Applanation tonometry was popularized by Goldmann as an improved method of intraocular pressure determination in comparison to indentation tonometry. The principal of Goldmann's applanation tonometry is based upon the Imbert-Fick principle, which teaches that the pressure inside a liquid-filled sphere can be determined by measuring the force required to flatten a portion of the surface. It will be obvious to one knowledgeable about the art that variations in thickness of the cornea would affect the accuracy of its applanation in the Goldmann technique. Specifically, a thinner than normal cornea would applanate easier than a normal thickness cornea, thereby generating a falsely low measure of intraocular pressure. Conversely, a thicker cornea than normal would overestimate the true intraocular pressure. Since the diagnosis of glaucoma and the assessment of the adequacy of treatment is largely dependent on intraocular pressure, the accuracy of intraocular pressure measurement is of paramount importance. In order to compensate for variations in corneal thickness, prior art has used pachymetry by optical or ultrasonic means to measure corneal thickness. It is time-consuming and expensive to use a second machine (e.g. ultrasonic pachymetry) sequentially. Moreover, it was impossible to know if the portion of cornea applanated for tonometry was the portion whose thickness was measured. Finally, the determination of both applanation tonometry and corneal pachymetry required solving an equation in order to identify the true intraocular pressure. As a result, the correction of applanation tonometry for corneal thickness variables is not generally done except in research circumstances. Recently, studies of ocular hypertensive patients have demonstrated that corneal thickness is the single most important predictor of glaucoma. Corneal thickness is inversely proportional to the risk of developing glaucomatous damage. That is to say, among ocular hypertensives, the thinner the cornea the greater the risk of glaucoma. In U.S. Pat. No. 6,083,161, I disclosed a new apparatus and method which provides more accurate intraocular pressure determination because using a single device it measures corneal thickness at the exact point of corneal applanation, then it corrects the measured intraocular pressure for the measured corneal thickness.
During standard applanation tonometry, a fluorescein-type dye is applied to the corneal surface. The operator looks through the oculars of the slit lamp in order to get a magnified view of the applanation device. The operator views the corneal surface in the area to be applanated by viewing through the transparent applanating component. This helps to reduce inadvertent trauma to the delicate epithelial layer of the cornea. The operator determines the endpoint of applanation by identifying a predetermined pattern of fluorescein dye created when just enough contact is made with the corneal surface. This is because to measure tonometry accurately, it is important not to under-applanate or over-applanate the cornea. My prior U.S. Pat. No. 6,083,161 teaches an improved apparatus and method of intraocular pressure determination but the invention made it more difficult for the operator to view the cornea during applanation in order to avoid traumatic contact and to view the pattern of the dye for determination of the endpoint of applanation. This viewing problem was the result of the incorporation of an opaque ultrasonic transducer in the applanating component. In order to be significantly more user friendly, the present invention simultaneously measures applanation tonometry and corneal pachymetry at the same point on the cornea and it allows for preservation of the operator's ability to visualize the corneal surface and the dye pattern during measurements by means of internal reflection.
My prior U.S. Pat. No. 6,083,161, upon related and earlier technology, described an applanation apparatus which allows for the determination of applanation pressure and membrane thickness of the eye, having a fluid filled cavity, wherein a transparent transducer body having a corneal contact surface for applanation of the cornea, utilizes an ultrasonic transmitter and receiver within said transparent transducer body for sending and receiving an ultrasonic signal to said applanated cornea, with the ultrasonic signal being processed to determine the applanation pressure and the membrane thickness of the eye.
In addition, the prior patent to Massie, U.S. Pat. No. 5,636,635, discloses a tonometer, that is a non-contact tonometer, for measuring the intraocular pressure of an eye, the cornea of the eye having a generally convex surface, which includes the use of an ultrasonic transducer means, that focuses a beam of acoustic radiation on a spot on the surface of the cornea, to produce sufficient radiation pressure to temporarily distort the surface of the cornea, it also utilizes a distortion detection means for detecting that deflected change in surface, and also utilizes acoustic radiation pressure estimating means for providing a signal from which the radiation pressure on the spot can be estimated, and then includes means for correlated said signal with the distortion to estimate the intraocular pressure of the eye.